The curious case of proton migration under pressure in the malonic acid and 4,4′-bipyridine cocrystal

Pressure was successfully used to induce single and double proton-transfer reactions in malonic acid and the 4,4′-bipyridine cocrystal. After contrasting with similar literature examples, an extended correlation between the ΔpK a values of coformers and the pressure necessary to initiate proton-transfer reactions is unveiled.


S2.1. XRD experimental tables
. Unit-cell parameters and order of the short experiments aimed at establishing the pressure limits for C2/c and P21/c phases.Each section of the table (separated by a thick black line) represent data for an individual sample crystal that was measured.Additionally, information about the phase is included (C2/c highlighted in purple and P21/c in orange).The ordinal numbers (O.N.) signify the order in which the crystal was measured.For all crystals the compression is considered a 'rapid' compression, as X-ray diffraction measurements were short (approx.3h) and sample did not remain under given pressure for longer than several hours.Table S3.The order of high-pressure experiments used for solving and refinement of the crystal structures.Each colour section of the table represent data for an individual sample crystal that was measured.Additionally, information about the phase is included.The ordinal numbers (O.N.) signify the order in which the crystal was measured.The 'slow' compression mode refers to samples that were compressed gradually from ambient pressure (or decompressed).The gradual nature was achieved by performing approx.24h long measurement at each pressure stage, letting sample to rest under specific pressure for at least 1 day.For 'rapid' compression mode the pressure was achieved directly without leaving sample crystal under lower pressures for a long period of time.Table S4.DFT results for the C2/c space group.Protons header describes the initial and final protonation states, e.g.0P→1P means that the initial guess structure is the neutral form and the final optimized structure contains one transferred proton from MA to BIPY.E/u.f. is energy per unit formula relative to the lowest energy of all computed structures.Table S5.DFT results for the P21/c space group.Protons header describes the initial and final protonation states, e.g.0P→1P means that the initial guess structure is the neutral form and the final optimized structure contains one transferred proton from MA to BIPY.E/u.f. is energy per unit formula relative to the lowest energy of all computed structures.a In case of oxalic acid dihydrate form α two values of pressure of proton transfer can be found in the literature.Original study by Casati et al.(2009) from 2009 reported formation of hydronium ion above 5.3 GPa (OXAAH2O).The reaction was hypothesized based on the changes in the geometry of carboxylic group of the acid and DFT calculations, and was later confirmed by neutron diffraction and spectroscopic studies.(Macchi et al., 2010) However, subsequent IR study by Bhatt et al.(2016) reported that the proton migration in oxalic acid dihydrate might actually start at much lower pressure of 2 GPa (OXAH2O-x), with pure ionic phase archived at approx. 5 GPa.Therefore, in the discussion in the main manuscript the value of 5.3 GPa was discussed as it corresponds to complete transformation from neutral to ionic phase.Interestingly, a study of form β of oxalic acid dihydrate did not lead to proton-transfer reaction, despite both polymorphs exhibiting quite similar geometries of the O-H⋯O hydrogen bonds at ambient conditions (with O⋯O distances equal 2.51 and 2.52 Å in form α and β, respectively) (Macchi et al., 2010).The different pressure response of polymorph β was explained by the less favourable orientation of the water molecule.
b The exact pressure for proton-transfer reaction (pPT) was not given by the authors; therefore the whole pressure range sample was investigated in is given.(Galiois et al., 1985), (Gallois et al., 1987), (Gallois et al., 1986 2); (g) 3.12(2); (h) 3.17(2); (i) 3.22(2) GPa, as well as at 298 K decompressed to (j) 2.76(2) GPa.Ruby chip is visible to the left of the crystal, and filter fibre holding the crystal is stretched across the opening of the gasket.During compression the opening of the gasket changed significantly therefore the scale was included for each figure section separately.
Figure S4.Raman spectra for gradual sample compression.Spectra were measured in order, from bottom to top.

Figure S5 .
Figure S5.Raman spectra for rapid sample compression (with spectra recorded for decompressed sample after each step).Spectra were measured in order from bottom to top.

Figure S6 .
Figure S6.Experimental frequency-pressure data of the main modes between 250 and 830 cm -1 (a), as well as 1000 and 1650 cm -1 (b).At around 2.85 GPa a slope change is observed.

Figure S7 .
Figure S7.Comparison of Raman modes from experiment and simulations for (a) cocrystal at 0 GPa and (b) salt at 3.14 GPa.

Figure S9 .
Figure S9.The C-O and C=O bond lengths, shown in red and blue, respectively, obtained from DFT calculations when all atoms in the MA and BIPY molecules were treated as symmetry-independent. Results of calculations in C2/c space group are shown on the left and in P21/c on the right.

Figure
Figure S10.The O•••N and C•••N distances, shown in red and blue, respectively, obtained from DFT calculations when all atoms in the MA and BIPY molecules were treated as symmetry-independent. Results of calculations in C2/c space group are shown on the left and in P21/c on the right.

Figure S12 .
Figure S12.Energy barrier for the proton transfer in the C2/c space group at 1150 Å 3 .

Figure S14 .
Figure S14.Pressure dependence of N•••C•••N and C•••C•••C angles of BIPYMA/BIPYH + MA -/BIPYH 2 2+ MA 2-chains shown in blue and red, respectively.Data for C2/c phase are shown with circles and diamonds, and for P21/c phase with squares and triangles.Empty symbols mark data for structures od decompressed crystal.The insert shows the manner each angle was measured.

Figure S15 .
Figure S15.Pressure dependence of the carbon-oxygen bond lengths in crystal structure of PYSUC.Authors of the original report did not provide the pressure values for each structure in the paper and in the CIFs, and therefore the pressure values were deduced from the Figure3of the original manuscript(Ward et al., 2023) assuming the numbering in the names of the structures reflect the order of the measurements on increasing the pressure.

Figure S16 .
Figure S16.Pressure dependence of the carbon-oxygen bond lengths in crystal structure of PYGLU.Authors of the original report did not provide the pressure values for each structure in the paper and in the CIFs, and therefore the pressure values were deduced from the Figure3of the original manuscript(Ward et al., 2023) assuming the numbering in the names of the structures reflect the order of the measurements on increasing the pressure.

Figure S17 .
Figure S17.Pressure dependence of the carbon-oxygen bond lengths in crystal structure of PYOX.

Table S1 .
Experimental details

Tables summarizing literature and CSD analysisTable S6 .
List of multicomponent crystals for which pressure-induced proton transfer reaction was reported in the literature and for BIPYMA.

Table S7 .
List of REFCODE families of multicomponent crystals (excluding polymers) studied under high-pressure and deposited in the CSD.Salts are highlighted in pink, cocrystals in green, clathrates in grey, hydrates in blue, solvates in orange and cocrystals of salts in purple.Two refcode families for structures for which proton transfer was reported are listed in red.

Table S8 .
List of selected multicomponent crystals investigated under high pressure, with crystal structures reported in the CSD, that contain acid-base pairs capable of undergoing proton-transfer reaction, but such reaction was not observed and with ΔpKa in -6 to 2 range.Crystals are named with the names of REFCODE families they belong to.Pressure range the crystals were investigated in (p) are also listed.

Table S9 .
Ward et al (2023)s of pyrazine with dicarboxylic acids studied under high-pressure conditions byWard et al (2023), including the pKa and ΔpKa values for the coformers, and the pressure range the cocrystals were